1. Field of the Invention
The present disclosure is related to obtaining a resistivity parameter in a wellbore environment. In particular, the present disclosure is related to obtaining apparent resistivity curves indicative of resistivity in the borehole, invasion zone and formation.
2. Description of the Related Art
Electromagnetic induction resistivity instruments can be used to determine the electrical conductivity of earth formations surrounding a wellbore. An electromagnetic induction well logging instrument is described, for example, in U.S. Pat. No. 5,452,761 issued to Beard et al. The instrument described in the Beard '761 patent includes a transmitter coil and a plurality of receiver coils positioned at axially spaced apart locations along the instrument housing. An alternating current is passed through the transmitter coil. Voltages which are induced in the receiver coils as a result of alternating magnetic fields induced in the earth formations are then measured. The magnitude of certain phase components of the induced receiver voltages are related to the conductivity of the media surrounding the instrument.
The development of deep-looking electromagnetic tools has a long history. Such tools are used to achieve a variety of different objectives. Deep looking tools attempt to measure the reservoir properties between wells at distances ranging from tens to hundreds of meters (ultra-deep scale). There are single-well and cross-well approaches, most of which are rooted in the technologies of radar/seismic wave propagation physics. This group of tools is naturally limited by, among other things, their applicability to only high resistivity formations and the power available down-hole.
Multi-frequency focusing (MFF) is an efficient way of increasing depth of investigation for electromagnetic logging tools. It is being successfully used in wireline applications, for example, in processing and interpretation of multi-component measurement devices. An example of such a device is the 3DExplorer® (3DEX®) induction logging instrument of Baker Hughes. In the 3DEX® instrument, three transmitters are placed axially on a tool mandrel and induce magnetic fields in three mutually orthogonal spatial directions: x, y, and z). The z-axis is chosen to be along the longitudinal axis of the tool, and the x-axis and y-axis are mutually perpendicular directions lying in the plane transverse to the axis. Three receivers, Rx, Rz, and Ry, are aligned along the orthogonal system defined by the transmitters. Measurements can be made for the corresponding magnetic fields Hxx, Hzz, and Hyy as well as cross-components, Hxy and Hxz. 3DEX® is operable in single frequency or multiple frequency modes.
Obtaining resistivity measurements of signals emanating from the borehole, the invasion zone and the formation can be useful in determining resistivity of a bed formation. Short spacing induction is useful in obtaining the near field measurements. There are several factors that make it difficult to perform short spacing induction measurements during logging. The most significant factors are the tool standoff and the rugosity of the borehole wall. The first way to reduce the parasitic effects of these factors is through reduction of the operating frequency. Unfortunately, this generally leads to loss of the signal strength. An alternative way is based on the dual frequency (or even multi-frequency) measurements which may lead to an even higher degree of signal loss. Therefore, there is a need to have a system that has improved immunity to the mentioned parasitic factors than the system based on frequency measurements. The present disclosure addresses this need.